US 7960743 B2
The invention relates to a broad-band light emitting diode having an active layer composed of a plurality of light emission regions of differing materials for emitting light at a plurality of wavelengths, wherein each of the emission regions of the active layer is electrically controlled by a separate electrode for providing a broad-band emission or optical gain with a multi-point control of its spectral profile.
1. A light emitting diode (LED), comprising:
a semiconductor heterostructure;
a waveguide formed in said semiconductor heterostructure and having an output facet;
a plurality of light emission regions of said semiconductor heterostructure disposed along the waveguide for emitting light thereinto, wherein each light emission region has a material composition for emitting light that is spectrally centered at a different light emission wavelength than other light emission regions; and,
a plurality of electrical contacts for electrically pumping each of the light emission regions separately, so as to produce individually controllable light emission at each of the light emission wavelengths.
2. A LED of
3. A LED of
4. A LED of
5. A LED of
6. A LED of
the electrical contacts are disposed over the cladding layer each in electrical communication with a different one of the light emission region; and,
the cladding layer comprises:
conducting regions for conducting electrical current from the electrical contacts through respective light emission regions, and
insulating regions for preventing the electrical current from each of the electrical contacts to flow through more than one of the light emitting regions.
7. A LED of
8. A LED of
9. A LED of
10. A LED of
11. A LED of
12. A LED of
The present invention generally relates to semiconductor light emitting devices, and more particularly relates to multi-electrode superluminescent semiconductor diodes with broad emission spectrum.
Semiconductor light emitting devices such as laser diodes, light emitting diodes and superluminescent diodes (SLD) are extensively used in many applications. Some of these applications require emitters that combine high brightness with a very broad optical spectrum, preferably as broad as tens or even hundredth nanometers (nm); these applications include the optical coherence tomography (OCT), low-coherence spectroscopy, and broad band optical amplification in an optical amplifier or tunable laser. For such application the superluminescent diodes, which provide optical amplification in the absence of lasing and are therefore characterized by a relatively high emission intensity in combination with a relatively broad optical spectrum, are typically used.
However, the emission bandwidth of a conventional SLD is limited by its material properties, which define the spectral shape and width of the SLD emission for a device of a given length. The emission spectrum of a typical SLD has a full width half maximum (FWHM) bandwidth Δλ of about 2-2.5% of its peak wavelength λ, so that a 1550 nm SLD would typically emit light having the FWHM bandwidth of about 35 nm, while such applications as low-coherence interferometers would benefit from a light source with a much broader spectrum, since their resolution is inversely proportional to the spectral bandwidth of the used light source.
A solution to this problem is proposed in U.S. Pat. No. 6,184,542 in the name of G. A. Alphonse, which discloses a multi-section GaAs based SLD shown in
Similar to a conventional SLD, the device of Alphonse has an n type cladding layer 3 that is deposited on a substrate 2, which is followed by an active layer 10 and a p type cladding layer 5. The refractive index of the active layer 10 is greater than the refractive index of the two cladding layers 3, 5 to provide a waveguiding effect in the direction normal to the layers. A capping layer 6 is deposited on the p type cladding layer 5. After the capping layer 6 is deposited, photolithography and etching is performed to define the waveguide as a ridge 8 with channels 9 on the sides. The channels are patterned and the capping layer 6 and the cladding layer 5 are etched down to an etch stop layer, not shown, within the cladding layer 5. Thus, in the channels, a small portion of the cladding layer 5 overlies the active layer 10. A base electrical contact 1 is then deposited to overlie the surface of the substrate 2 opposite the n type cladding layer 3. The electrical contact 1 is an alloy including one or more of germanium, gold, and nickel. A dielectric is then deposited over the entire top surface of the structure. Using photolithography and etching, a stripe is opened over the ridge 8, and a metal, such as an alloy of titanium, platinum, and gold, is deposited therein on the capping layer 6 as a top electrical contact 7 in the stripe region to confine electrical current to the ridge region. The waveguide stripe of
To broaden the SLD emission spectrum, the active layer 10 is formed of three neighboring emission layers 15, 20, 25 of differing material composition aligned along the length of the device as illustrated in
However, we found that an SLD of the type described in the '542 patent tends to have an un-even, rather than flat, emission spectrum that is difficult to control and that is typically less spectrally broad than expected on the basis of the bandgap spread of the used material, e.g. less broad than combined radiation from three conventional SLDs with the differing center emission wavelength λ1, λ2, and λ3.
An object of the present invention is to overcome at least some of the deficiencies of the prior art by providing a multi-section semiconductor light emitting device having a plurality of light emission regions that are individually addressable electrically for emitting broad-band light with controllable spectral profile.
The invention relates to a broad-band light emitting diode having an active layer composed of a plurality of light emission regions of differing materials for emitting light at a plurality of wavelengths, wherein each of the emission regions of the active layer is electrically controlled by a separate electrode for providing an broad-band optical emission or broad-band optical gain with a multi-point control of the respective spectral profile.
In accordance with the invention, there is provided a light emitting diode (LED), comprising: a semiconductor heterostructure; a waveguide formed in said semiconductor heterostructure and having an output facet; a plurality of light emission regions of said semiconductor heterostructure disposed along the waveguide for emitting light thereinto, wherein each light emission region has a material composition for emitting light that is spectrally centered at a different light emission wavelength than other light emission regions; and, a plurality of electrical contacts for electrically pumping each of the light emission regions separately, so as to produce individually controllable light emission at each of the light emission wavelengths.
In one aspect of the invention, insulating regions are disposed in the semiconductor heterostructure between the electrical contacts for electrically insulating adjacent electrical contacts from each other.
One aspect of the present invention relates to the LED wherein the semiconductor heterostructure comprises a cladding layer disposed over the plurality of light emission regions, wherein a plurality of electrical contacts are disposed over the cladding layer each in electrical communication with a different one of the light emission region, and wherein the cladding layer comprises conducting regions for conducting electrical current from the electrical contacts through respective light emission regions, and insulating regions for preventing the electrical current from each of the electrical contacts to flow through more than one of the light emitting regions.
According to one aspect of the invention, the plurality of light emission regions includes at least two light emission regions of which a region further away from the output facet extends along the waveguide at a greater length than the region that is closer to the output facet.
The invention will be described in greater detail with reference to the accompanying drawings which represent preferred embodiments thereof, wherein:
The prior art SLD having multiple emission layers of differing center emission wavelengths was discovered to have a significant limitation, which stems from an interdependence of electrical and optical emission properties of semiconductor heterostructures with p-n junctions, and which makes it difficult or even impossible to fully realize the potential bandwidth enhancement provided by the multi-section active layer.
This limitation of the prior art multi-section SLD will now be explained with reference to
One disadvantage of the SLD 30 is that the differing band gaps of the emission regions 51-54 result in differing resistivities thereof for any given voltage V that is applied to the electrode 60, yielding an inhomogeneous flow of the electrical current through the emission layers 51-54. This inhomogeneous current flow in the SLD 30 is schematically illustrated with arrows 71-74 of differing lengths representing differing electrical currents J′1<J′2<J′3<J′4 which flow through the sections 51-54. Here, J=J′1+J′2+J′3+J′4, with comparatively stronger currents J′3, J′4 flowing through sections 53, 54 having narrower band gaps and thus having larger center emission wavelengths 3, 4, respectively.
Turning now to
The present invention improves upon the prior art by providing a semiconductor light emitting diode (LED) that is free from the above described limitation. The term “LED” as used herein includes both conventional light emitting diodes that operate without light amplification in their active regions and superluminescent diodes (SLD) which in operation exhibit significant positive optical gain in the active region of the device without lasing. The improved LED is comprised of a semiconductor heterostructure with a waveguide formed therein, with a plurality of light emission regions of differing materials and central emission wavelengths disposed along the waveguide for emitting light thereinto, and further having a separate dedicated electrical contact, or electrode, for electrically pumping each of the multiple emission regions separately with a suitable electrical current, so as to produce individually controllable light emission at each of the light emission wavelengths. Means for electrically isolating each of these electrodes from all but a respective one of the emission regions may further be provided. By applying generally differing individually controlled voltages to each of these electrodes, the electrical currents flowing through each of the multiple emission layers of the LED can be individually adjusted to provide a desired spectral profile of the overall LED emission.
These and other features of the present invention will now be first described with reference to
Similarly to the SLD 30, the LED 100 is based upon a semiconductor heterostructure 150 having the upper and lower cladding layers 40, 35 of opposite conductivity types, i.e. p and n, sandwiching the active layer 50 that is composed of the four butt-coupled emission regions 51-54 disposed in series between the AR-coated front facet 80 and the reflecting back facet 90. Each emission region 51, 52, 53 and 54 has a different center emission wavelength λ1, λ2, λ3, and λ4, respectively, as indicated in the figure. The cladding layers 40 and 35 have a high electrical conductivity suitable for enabling electrical current flow through the active layer 50 without substantial resistance, as is common in light-emitting semiconductor diodes.
In accordance with the invention, the semiconductor heterostructure 150 includes four separate electrodes 101-104, each associated with and disposed over a different emission region 51-54 and separated from neighboring electrodes by gaps, for electrically pumping each of the light emission regions 51-54 separately. The semiconductor heterostructure 150 preferably further includes insulation regions 120 that are disposed in between the electrical contacts 101-104 in the top cladding layer 40 to electrically insulate these contacts from each other. With these isolation, the electrical contacts 101-104 can be electrically biased at differing voltages independently of each other, for passing individually controllable electrical currents J1, J2, J3 and J4 111-114 through the emission layers 51, 52, 53 and 54, respectively. Accordingly, the LED 100 may be considered as composed of four emission sections formed in the single heterostructure 150 and butt-coupled in series between the front and back facets 80 and 90, each including one of the emission regions 51-54 and a corresponding electrical contact 101-104, respectively; these sections are indicated in
In operation, each of the electrodes 101-104 is used for independently biasing the respective emission region 51-54 so as to and feed each of these regions with its own electrical current Ji, where i=1, . . . , 4, or, generally, i=1, . . . , N, as schematically shown in
When the same voltage V=V′ is applied to each of the electrodes 111-114 of the LED 100, or, equivalently, this voltage is applied to the single electrode 70 of the SLD 30, electrical currents J′i flowing through the emission regions 51-54 all have different values, as illustrated in
Advantageously, the electrical separation and isolation of the electrodes 101-104 from each other in the LED 100 of the present invention enables biasing of the emission regions 51-54 according to their respective energy band gaps. For example,
Turning now to
In an embodiment wherein the materials and band gap energies of the four emission regions 51-54 are such that the corresponding central emission wavelengths λ1, λ2, λ3, and λ4 are spaced apart according to the FWHM bandwidths Δλi of the respective emission spectra 161-164 as shown in
In embodiments wherein the LED 100 is designed for use as a broad-band light source with controllable spectral profile, the different emission regions 51-54 are preferably arranged in such a way that the light emitted by each of these section into the waveguide towards the output facet 80 is not re-absorbed by the next section on its way to the output facet 80. To achieve this, the emission regions 51-54 are preferably disposed in ascending order of the center emission wavelengths from the output facet 80 on along the waveguide, or equivalently in order of increasing energy band gap values towards the output facet 80. In order to optimize the device and assist in achieving a flat emission spectrum, it may be advantageous to vary the lengths of the emission regions 51-54, that is make some of the emission regions longer or shorter than others. For example, emission regions with relatively narrower band gaps may be made longer than the emission regions with the wider band gaps, since they may experience more attenuation within the device. For example, light emitted by the smaller bandgap section 53 experiences additional optical losses as it travels through the wider band-gap sections 52 and 51 to reach the output facet 80. Increasing the length of the emission region 53 along the optical path of light in the active layer 50 enables increasing the current 114 in the section and therefore increasing the amount of light emitted by the section 54 from the output facet 80 of the LED 100 for the same applied voltage. Generally, the LED 100 may include at least two light emission regions of which a region having a greater light emission wavelength extends along the waveguide at a greater length. Alternatively or simultaneously, the LED 100 may include at least two light emission regions of which a region that is further away from the output facet 80 extends along the waveguide at a greater length than the region that is closer to the output facet. In some embodiments, the emission region 54 that is adjacent to the reflecting back facet 90 may be made shorter than one or all of the other emission regions of the LED 100 as light emitted by this region towards the back facet 90 is not lost but reflected towards the front facet 80.
The semiconductor heterostructure 150 utilized in the LED 100 may be composed of alloys of different semiconductor materials as known in the art including but not limited to AlGaAs/Gas, InGaAsP/InP, InAlGaAs/InP heterostructures, which may be grown on GaAs and InP substrates by suitable epitaxial techniques such as LPE (liquid-phase epitaxy), MBE (molecular beam epitaxy) and MOCVD (metallo-organic chemical vapor deposition). The InGaAsP/InP material system in particular is well suited to the fabrication of multi-sections devices and can also emit light over a wide range of wavelength, for example from 1100 nm up to 1650 nm, by adjusting ratios of the different component In, Ga, As and P.
The active layer 50 within each of the emission regions 51-54 may be comprised of multiple quantum wells (MQWs) or be a bulk material layer, depending whether a broad spectrum or a high spectral density of light emission is favored for a particular. When disposed between the oppositely doped cladding layers 35 and 40, the active layer 50 forms a p/n heterojunction therewith, preferably of a p/i/n (p-doped/non-doped/n-doped) type which is suitable for injecting and confining electrical carriers in the active layer 50. To achieve this, the cladding layers 35, 40 each have a wider bandgap than the active layer 50 in any of the emission regions 51-54, so as to confine the carrier injected therein. By way of example, the upper cladding layer 40 may be p-doped to a doping level between about 1017 and 1.5×1018 cm−3 and the lower cladding layer 35 may be n-doped to a doping level between 1017 and 2×1018 cm−3, with the material of the active layer 50 undoped so as to form the p/i/n junction. The wider bandgap materials that constitute the doped cladding layers 35, 40 preferably have lower refractive indices than the active layer 50 in order to form a single-mode optical waveguide in the direction normal to the plane of the layers that efficiently confines and guides the light in the active layer 50.
Referring now to
The active layer 250 may be about 200 nm thick and is composed of three different InGaAsP alloys, with differing proportions of its constituent elements In, Ga, As, and P in the emission regions 251-253, which are aligned adjacently to each other along the optical path. Particular material compositions of the emission regions 251-253 are selected so as to provide three different band gap values according to desired center emission wavelengths λ1, λ2, and λ3 of the light emission from these regions 251-253. The light emission regions 251-253 may be disposed in ascending order of the center emission wavelengths from the output facet 80 along the waveguide, so that light from emission regions of a comparatively wider band gap and therefore smaller wavelength would not have to propagate through an emission region of a comparatively narrower bandgap on its way to the output facet 80.
The upper cladding layer 240 that is disposed over the undoped active layer 250 may be about 1.5 um thick and composed of p-doped InP that is doped with an appropriate acceptor doping material such as Zn to provide acceptor concentration on the order of 5×1017 cm−3.
A thin contact layer 210 that is heavily p-doped is disposed over the upper cladding 240; it may be about 100 nm thick and comprised of In0.53Ga0.47As1 doped with a suitable dopant such a Zn to an acceptor concentration of 1×1018 cm−3 or higher. Three separate electrical contacts 201-203 are disposed over the contact layer 210 in electrical communication with one and only one of the light emission regions 251-253, and are preferably in a vertical alignment with the corresponding one of the emission regions 251-253, as illustrated in
The substrate 260 has a metalized lower surface that forms a base electrical contact 270 that is common for all light emission regions 251-253; for this contact, a suitable n-type metallization such as the metal alloy Ti (200 nm)/Pt (200 nm)/Au (200 nm) may be conveniently used.
By way of example, the material composition of the first emission region 251 may be In0.75Ga0.24As0.51P0.48 resulting in operation in a first ASE spectrum having the first central emission wavelength λ1 of about 1250 nm, material composition of the second emission region 252 may be In0.66Ga0.34As0.71P0.29 resulting in a second ASE spectrum having the second central emission wavelength λ2 of about 1400 nm, and material composition of the third emission region 253 may be In0.59Ga0.4As0.87P0.13 resulting in a third ASE spectrum having the third central emission wavelength λ3 of about 1550 nm. When suitable voltages are applied to the electrical contacts 201-203 so as to individually bias the respective emission regions 251-253 at pre-determined differing levels corresponding to their band gap values, the LED 200 emits light beam 265 from the output facet 80 having an optical spectrum with a total FWHM spectral width in excess of 300 nm. It will be appreciated that other compositions of the emission regions 251-253 of the active layer 250 are also possible, depending on the desired values of the central emission wavelengths λ1, λ2, and λ3 and the desired spectral shape of the output light 265.
Turning now to
The front facet 80, which is typically cleaved but may also be etched, is preferably coated with a suitable anti-reflection coating as known in the art in order to avoid the appearance of undulations in the emission spectrum of the LED 200 associated with an optical cavity. The back facet 90 may be either antireflection coated if the LED 200 is to be utilized as a wide-band amplifier, or high-reflection coated to force most of the light generated in the emission regions 251-253 to be emitted by the front facet 80 if the LED 20 is to be used as a broad-band emission source.
The waveguide 230 is preferably tilted with respect to the output facet 80 in the plane of the heterostructure 350 and the substrate 260, so as to further reduce the amount of light that can be reflected back into the waveguide 230 by the facets in order to eliminate ripples in the spectrum of the emitted light 265 due to a residual cavity effect. The tilt angle 270, that is as an angle between a line normal to the front facet 80 and an optical axis of the waveguide 230 indicated with arrows “B”, may for example be between 5 and 10 degrees. The two arrows “B” also indicate the plane of the vertical cross-section of the LED 200 that is shown in
The electrical contact pads 201-203 are shown disposed over the waveguide 230 in vertical alignment with the emission regions 251-253, and are physically and electrically separated from each other by the insulation regions 220.
Referring now to
Referring now to
The LED 200 can be fabricated using known semiconductor processing techniques that are commonly used in different combinations in fabrication of edge-emitting semiconductor lasers, semiconductor amplifiers and conventional light-emitting diodes. The fabrication process includes multi-step epitaxial re-growth since each section needs a different material composition in at least the active layer 250. MOCVD is one growth technique that can be used to growth the different layers of material, but MBE can also be used. Each growth step is followed by a selective etching step where part of the previously grown active layer 250 is removed to provide an area to grow a next emission region. The different emission regions are therefore “butt-coupled” to each other allowing the light to travel from a section to another with almost no optical losses. Once all the sections of the active layer are grown, the top cladding layer of lower refractive and wider bandgap semiconductor, for example InP, is grown over all sections. The waveguide is then formed using successive lithography, etching and re-growth steps. The electrical isolation between the sections may be obtained by first introducing a proton (H+) implantation in the cladding layer between the sections and by removing the metallic electrode as well as the highly doped contact layer between the sections. More than 1 Mega Ohms of electrical insulation can be achieved using this configuration. The devices are then cleaved and the anti-reflection or reflective coatings are deposited on the cleaved facets. Next the device is soldered on a carrier that provides thermal dissipation (Copper, Aluminum nitride or Copper/Tungsten or Silicon).
The fabrication steps outlined hereinabove will now be described in further detail by way of example as applied to the LED 200 and with reference to
Turning first to
Next, the first emission material 251 is etched away everywhere but in the first emission region, as shown in
Next, the structure is covered with a new protective layer 406 as shown in
Once all three emission regions 251-253 of the active layer 250 are formed with their respective emission materials, the top cladding layer 240 and the contact layer 210 are sequentially grown over the whole structure, as illustrated in
Next, the structure is patterned with a photoresist 430 or other suitable masking material as shown in
Next, the photoresist 430 is stripped resulting in a structure illustrated in
Following the formation of the ridge waveguide 230, a p-type metal contact layer 440 is deposited over the structure in the area of the waveguide 250 as illustrated in
Finally, the structure is cleaved to form the front and back facets 80, 90 of the LED 200, which are then coated with the respective coatings 81, 91 as known in the art.
In operation, bias voltages Vi, i=1, 2, 3, of the proper polarity are individually applied to the electrical contacts 201-203 of the LED 200 to produce light emission in all three emission regions 501-503. The light emission from the emission regions 501-503 is individually controlled, thereby resulting in the output light 265 from the LED 200 that has the desired spectral profile, which may be substantially flat and broad-band. The individual control of the electrical currents 211-213 that pump the emission regions 251-253 enables to independently control the spectral intensity of the output light 265 in each of the ASE frequency bands about the central emission wavelengths λ1, λ2, and λ3, thereby providing light having a spectral profile that can be controllably adjusted to a desired shape.
Although the invention has been described with reference to specific embodiments for the purpose of illustration, it should be understood that each of the preceding embodiments of the present invention may utilize a portion of another embodiment; persons skilled in the art will be able to modify these embodiments according to their specific requirements, and other embodiments are also possible within the scope of the invention. For example, the conductivity types of the described layers can all simultaneously be changed to their opposites, and additional layers may be present in the structure. Furthermore, although the specific sequences of the emission regions described hereinabove are advantageous for application wherein the LED of the present invention is used as a broad-band light source, in other applications such as broad-band light amplification a symmetrical arrangement of the light emission regions can be beneficial wherein the emission regions adjacent to the front and back facets are composed of a material of equal bandgap, with emission regions having a smaller bandgap disposed away from the facets in the middle of the device. An ordinary person in the art would be able to construct such embodiments without undue experimentation in light of the present disclosure.
Of course numerous other embodiments may be envisioned without departing from the spirit and scope of the invention.